taken 7 years ago, near to Antrim, Northern Ireland

One of three bench marks in the porch of All Saints Parish Church in Antrim - seen in J1486 : Riverside, Antrim (1). This is the flush bracket, sited c1956 as part of the modern geodetic levelling of Northern Ireland carried out by the Ordnance Survey of Northern Ireland. This mark lies on the primary levelling line between Belfast and Magherafelt.
The number on the bracket is OSBM 3279.
The mark was originally levelled to 72.1 feet above MSL; it is now levelled to 21.98 metres above MSL.
There are two other bench marks in the porch of the church - see J1486 : Bench Mark, Antrim and J1486 : Bench Mark, Antrim. See also Link for many other examples I have found.

Bench marks were historically used to record the height above sea level of a location as surveyed against the Mean Sea Level data (taken at Clarendon Dock, Belfast, for Northern Ireland data, Newlyn in Cornwall for data in Great Britain and Portmoor Pier, Malin Head, for data relating to the Republic of Ireland). They were used as part of a greater surveying network by the UK Ordnance Survey, Ordnance Survey Northern Ireland (OSNI) and the Ordnance Survey Ireland (OSI). If the exact height of one bench mark is known then the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. In this way hundreds of thousands of bench marks were sited all around the UK & Ireland from the mid 19th to late 20th centuries. There are several distinct types of bench mark:

Bench marks are commonly found on older buildings or other semi-permanent features such as stone bridges or walls. Due to updated mapping techniques and technological advances such as GPS, bench marks are no longer maintained. Many are still in existence and the markers will probably remain until they are eventually destroyed by redevelopment or erosion.